Amyotrophic lateral sclerosis (ALS) is a devastating disease affecting about 2 in 100,000 people in the USA each year. Recent discoveries have the potential to dramatically change our understanding of the causes of ALS and identification of possible targets for treatment. ALS results from the selective loss of motor neurons, which in ALS patients contain cytoplasmic aggregates of proteins. Identification that a major constituent of these aggregates is the RNA-binding protein TDP-43, which is normally localized in the nucleus, has opened up many new directions in ALS research. A large number of mutations in TDP-43 have also been identified in ALS patients providing a causative link between this protein and the disease. TDP-43 aggregates have also been identified in post-mortem samples of patients who have died of other neurodegenerative diseases such as fronto-temporal dementias, Alzheimer's and Parkinson's diseases, suggesting that TDP-43 pathology might be a causative agent in a wide variety of diseases. It is critical to understand how TDP-43 dysfunction leads to disease. Two possible mechanisms have been proposed. The first is that the TDP-43 aggregates are inherently toxic and lead to motor neuron cell death. The second model, which is not mutually exclusive with the first, is that the loss of normally functioning protein in the nucleus is the proximate cause of motor neuron disruption. We are using the fruit fly, Drosophila melanogaster, as a model to understand the function of TDP- 43 in neurons and the consequences of aberrant expression. Null mutations in the Drosophila orthologue of TDP-43, named TBPH, are pupal lethal and show larval motor defects. Over-expression of any of three naturally occurring isoforms of TBPH in all tissues is early larval lethal. When expression is restricted to motor neurons, both larvae and adult flies have motor deficits. The bulk of the TDP-43 mutations in ALS are found in the C-terminal domain and our data shows that the C-terminal domain of TBPH is necessary for maximum toxicity. The primary goals of this application are to determine the cellular and molecular mechanisms underlying these defects. We have identified a genetic interaction between TBPH/TDP-43 and another neurodegeneration gene, named swiss cheese (sws), which suggests that sws is required for the toxic properties of TDP-43. This interaction is particularly intriguing because mutations in the human orthologue of sws, named neuropathy target esterase, lead to motor neuron disease. In addition to testing our hypothesis that the toxic effects of over-expressed TDP-43 result from an interaction between TDP-43 and sws we also propose to examine the anatomical and electrophysiological phenotypes of motor neurons that over-express TDP-43 in wild-type and sws mutant backgrounds. Any therapeutic approaches to treat ALS that rely on targeting TDP-43 dysfunction require an understanding of the molecular mechanisms that underlie the toxic effects of TDP-43. Using the powerful genetic tools available in Drosophila will greatly accelerate this understanding. PUBLIC HEALTH RELEVANCE: The first step in developing therapies for ALS is to identify the genes/proteins that either directly cause ALS or confer susceptibility to develop ALS in response to environmental triggers. Recent discoveries that strongly link TDP-43 to ALS suggest that this is a possible future target. Any therapeutic approaches to treat ALS that rely on targeting TDP-43 dysfunction require an understanding of the molecular mechanisms that underlie the toxic effects of TDP-43. This application focuses on understanding the mechanisms that lead to neurodegeneration by TDP-43 dysfunction using the fruit fly, Drosophila melanogaster as a model, an approach that will greatly speed up the process of designing therapies targeting TDP-43.